Recently, with advancement of making electric cables requiring an extra-high voltage and a large capacity, it has been desired to miniaturize the cables and to reduce dielectric loss for the purpose of reducing the cost of construction and the cost of operation. As a means for attaining these requirements, there have been improvements with respect to the insulating materials used. It has become necessary to use materials which not only have excellent electric properties such as dielectric breakdown strength to impulse voltage and to AC voltage or dielectric loss tangent, etc. but also excellent mechanical strength. To meet these requirements attempts have been made to utilize a plastic film instead of the previously used oil-impregnated paper composed of a natural cellulose paper and an insulating oil. However, this material has been found to be undesirable due to temperature dependent characteristics inherent in plastics, large polar effects on impulse voltage, deterioration of a withstand voltage to repeated application of voltage, difficulty of following a variation of excess oil pressure caused by inferior stream of oil between film layers, and other defects. In order to compensate for these faults so as to give a barrier effect for the impulse voltage and to keep the stream of oil between insulating layers, a compounding technique has been developed wherein a plastic film and cellulose fiber papers are united. Plastic films used for such a technique, must have excellent properties with respect to impulse voltage characteristic, AC voltage characteristic and dielectric characteristics, etc. Giving consideration to these factors polyolefin films have been found to be suitable. However, compound materials composed of polyolefin films and insulating papers become thicker when impregnated with oil as compared with only using the insulating paper because of swelling caused by the oil used (mineral oil, DDB, and hydrocarbon oils such as alkyl-naphthalenes, etc.), resulting in that the insulating paper present between films is pressed against the films which increases the unevenness of the surface or increases the resistance of oil streaming through the fibers. Consequently, the streaming of oil in the radial direction of the cable causes damage to the mechanical characteristics of insulating materials which are required for bending the cable or deteriorates other characteristics. In order to restrain the increase in thickness due to the swelling of the polyolefin films as much as possible, it is preferred to use polyolefin films which cause less variation in thickness. However, it is not possible to avoid an increase in thickness under existing circumstances due to physical limitations.
Even though essential restriction of swelling is impossible, it is possible to restrain an apparent increase in thickness caused by swelling. Techniques to accomplish this have been recently developed. One such technique comprises previously moistening insulating papers in the insulating material composed of a plastic film and insulating papers by utilizing a hygroscopic swelling property of the insulating papers to increase the thickness, winding the insulating material on a cable conductor, drying it to reduce the thickness of the insulating paper between plastic films, and impregnating the insulating papers wth oil. This technique is described in Japanese Patent Publication No. 38237/77. Thus, the increase in thickness of the plastic film by swelling can be compensated by the latitude of the insulating papers having a reduced thickness and, consequently, streaming of insulating oil is not prevented, even if the film is in a swollen state after being impregnated with insulating oil.
Another method comprises compulsorily forming prescribed unevenness on a compound material composed of a plastic film and fiber paper, namely, embossing, by which the increase in thickness caused by swelling of the plastic film after being impregnated with insulating oil is absorbed by deformation of the unevenness. However, in this method, there is a possibility of deterioration of mechanical and electric strengths, because the compound material composed of a plastic film and fiber papers is easily damaged locally during the mechanical embossment processing.